isUndefined()

isUndefined(value): value is undefined

Checks if a value is undefined.

note

Only checks undefined, not null. Use isNil to check both.

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Parameters

value: unknown

The value to check.

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Returns: value is undefined

true if the value is undefined, false otherwise.

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See Also

• isNil
• isNull
• isNonUndefined

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Since

1.0.0

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Example

isUndefined(undefined); // => true
isUndefined(void 0);    // => true
isUndefined(null);      // => false
isUndefined('');        // => false
isUndefined(0);         // => false

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How it works?

Type guard that checks if a value is undefined.

Type Narrowing

Common Checks

Value	Result
undefined	true
void 0	true
null	false
0	false
''	false
false	false

vs isNil

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Use Cases

Detect missing properties 📌

Check if a value is strictly undefined. Essential for checking optional parameters or missing object keys.

#detect#undefined#missing#optional#defaults#parameters

if (isUndefined(options.verbose)) {
  options.verbose = true; // Set default
}